Fix build error

[ILab-01]

I think we need "#include <unistd.h>" to avoid build error about usleep().
And I think we need "-lboost_system" in "Examples/ROS/ORB_SLAM2/CMakeLists.txt" to avoid link error.

This is my first pull request experience. Thank you.

[humanoid2050]

I concur with the fix, but I might suggest ${Boost_SYSTEM_LIBRARY} instead of the direct -lboost_system.

[ILab-01]

Thank you for your comment.
I add ${Boost_SYSTEM_LIBRARY} instead of -lboost_system.

[s-hemer]

Just ran across that issue too and did the same fixes. In principle, it would have been better to separate the boost system and include changes into two commits. But anyway, +1 for accepting this request !

[s-hemer]

By the way, #628 seems to address the same issue. So maybe you should thing of doing the include at the System.h and manually only there where that header is not included. Not sure which one is the better style.